Morphometric classification of kangaroo bones reveals paleoecological change in northwest Australia during the terminal Pleistocene

Specimen identification is the backbone of archeozoological research. The challenge of differentiating postcranial skeletal elements of closely related wild animals in biodiverse regions can prove a barrier to understanding past human foraging behaviours. Morphometrics are increasingly being employed to classify paleozoological animal remains, however, the potential of these methods to discriminate between wild animal groups has yet to be fully realised. Here we demonstrate the applicability of a traditional morphometric approach to taxonomically classify foot and ankle bones of kangaroos, a large and highly diverse marsupial family. Using multiple discriminant analysis, we classify archaeological specimens from Boodie Cave, in northwest Australia and identify the presence of two locally extinct macropod species during the terminal Pleistocene. The appearance of the banded hare-wallaby and northern nail-tail wallaby in the Pilbara region at this time provides independent evidence of the ecological and human responses to a changing climate at the end of the last Ice Age. Traditional morphometrics provides an accessible, inexpensive, and non-destructive tool for paleozoological specimen classification and has substantial potential for applications to other diverse wild faunas.


Large macropods
The main separation among astragali of large macropods is along Principal Component (PC) 2. This is driven by the breadth of the navicular facet, followed by differences in the proportions of the talocalcaneal facets on the plantar surface of the astragalus (Fig. S1). The astragali of agile wallaby, common wallaroo and the northern nail-tail wallaby exhibit a broad navicular facet, a shallow and narrow medial talocalcaneal facet and a short lateral talocalcaneal facet. In contrast red and grey kangaroo astragali have a deep and broad medial talocalcaneal facet and a longer lateral talocalcaneal facet. Calcanea of agile wallaby and northern nail-tail wallaby are separated from the other large macropods along PC1 by a more deeply stepped calcaneocuboid articulation and a broader sustentaculum tali for passage of the flexor digitorum longus (Fig. S2). Grey and red kangaroos and the wallaroos have a broader tuber calcanei and a longer lateral calcaneotalar facet. Red and grey kangaroos are separated from the wallaroos along PC2 by a more deeply stepped calcaneocuboid articulation and more slender tuber calcanei.

Supplementary Figure S2. Shape variation between calcanea of large macropods.
Fourth metatarsal shape of large macropods overlaps substantially, although some difference can be observed along PC1 associated with metatarsal length and robusticity of the metatarsal shaft. Red and grey kangaroos have longer, more mediolaterally slender metatarsals, while wallaroos, agile wallabies and northern nail-tail wallabies have shorter, more robust metatarsals (Fig. S3).

Medium macropods
The medium sized rock-wallaby astragali are separated from spectacled hare-wallabies and the nail-tail wallabies along PC1 by a longer lateral talocalcaneal facet and a broad navicular facet (Fig. S4). The astragali of spectacled hare-wallabies and nail-tail wallabies have a deeper medial talocalcaneal facet and broader trochlear crests than rock-wallabies. While astragali of both species of nail-tail wallaby are similar in shape to spectacled hare-wallabies the former are substantially larger in size.

Small macropods
The astragali of bettongs have a narrower lateral talocalcaneal facet, broader medial talocalcaneal facet and broader navicular facet than rufous and spectacled hare-wallabies (Fig. S7). Banded hare-wallaby astragali are intermediate in shape between the bettongs and other hare-wallabies in the main variation of the PCA and share similar features with both. Astragali of juvenile small macropods tend to score negatively along PC2 which is strongly associated with a longer talar neck.
Supplementary Figure S7. Shape variation between astragali of small macropods. Arrows indicate direction of measurement change between genera.
Bettong calcanea are separated from the other small macropods along PC1 by a narrow tuber calcanei, and broad dorsolateral calcaneocuboid articulation and broad medial calcaneotalar facet (Fig. S8). Conversely hare-wallabies and the Nabarlek have a broad tuber calcanei, a narrow medial calcaneotalar facet and long medial process on the sustentaculum tali. The shape of hare-wallaby calcanea vary from the Nabarlek along PC2 with the latter exhibiting a longer lateral calcaneotalar facet and shorter calcaneocuboid step.
Supplementary Figure S8. Shape variation between calcanea of small macropods. Arrows indicate direction of measurement change between genera.
Bettong and banded-hare wallaby fourth metatarsal shape overlaps entirely in the main variation of the PCA and are visually very similar in shape (Fig. S9). Rufous hare-wallaby metatarsals can be differentiated from other small macropods by their longer and more slender shape. Narbalek metatarsals are intermediate in shape to the rufous hare-wallaby and bettongs. Although proportionally similar, the proximal end of bettong fourth metatarsals is distinctive, exhibiting a 'L' shaped articulation while other small macropods have a more triangular proximal articulation (Fig. S9).  Principal component scores comprising 95% of total shape variation were used as a proxy for bone shape. Groups with < n-2 observations (where n is the number of principal components) were not tested. Bold indicates significant p-value (<0.01).